I had some work done on my heater and the tech checked the
air flow. He found that one of the 2 air return ducts was
smaller than it should have been. So I had a man come out
to give me an estimate on putting in a larger duct. His
proposal was to replace the 10" metal duct with a 14" flex
duct. I looked at it and the very idea of flex duct seemed
to be kind of iffy. So I did some research and found that
the general consensus is that flex duct is ok, if it is
installed right. And hat there is a lot of flex duct that
is not installed right. That involves mostly short straight
sections. Well, my installation involves a right angle
bend and a 1 foot offset near the end. Not exactly straight.
It took some doing but I finally got the salesman to understand
that it is going to be hard to get the duct 'fully extended'
with that much play in it. So you would get a lot of
turbulence and reduced flow. He thought that by oversizing
it from 12 to 14 inches it would make up for it. But
he did come around after a while. So they will put in
12 inch metal duct.

It costs more, but I figure you can do it cheap or you
can do it right.

In my search the site that I found to be about the best
was http://askweldin.com/Flex.html. There were a number
of places but I thought he provided the best over view.

Bill